In recent years, with the improvement of communication infrastructure, large-volume data such as music data and video data have been transmitted/received using the Internet. In such a circumstance, because of increasing demand for an environment where high-speed data communication is available in general households, an Internet connection environment by a communication system such as ADSL (Asymmetric Digital Subscriber Line) system using public telephone lines and by a cable connection system using a communication line of a cable television network has become widespread.
In the cable connection system, a mainline network and branch lines to households were conventionally realized by coaxial cables. However, the tendency has been toward an increase in use of data communication such as Internet connection as previously described. On this account, in the case of a best-effort connection system, there is a the problem that a heavy traffic causes a significant drop of a communication speed for each user.
In order to increase the capacity of data communication in a cable network, the introduction of the HFC (Hybrid Fiber/Coaxial) system using optical fibers for the part of a mainline network has been developed. This makes it possible to provide users with broadband data communication services of several Mbit/sec even at heavy traffic hours.
For example, even in the case of using 64QAM (Quadrature Amplitude Modulation), which is no longer state-of-art technology, as a modulation system of data communication used in the cable network, it is possible to carry out high-speed data communication with a bandwidth of 6 MHz and a transmission rate of 30 Mbit/sec. In fact, using an unassigned channel of a cable television, a transmission rate of 4 Mbit/sec to 27 Mbit/sec has been realized to use in transmitting/receiving digital signals over a cable network as a high-speed analog communication network in such a manner like a cable modem.
Hereinafter, a conventional tuner of a cable modem will be described with reference to a block diagram shown in FIG. 8. The cable modem is composed of a tuner, a digital signal processing circuit, and other components, and FIG. 8 illustrates a schematic structure of the tuner.
The tuner for use in a cable modem includes, for example, a receiver circuit for a UHF band (B3 band) covering 470 MHz to 860 MHz, a receiver circuit for a VHF High band (B2 band) covering 170 MHz to 470 MHz, and a receiver circuit for a VHF Low band (B1 band) covering 54 MHz to 170 MHz.
The cable line is connected to the input terminal 101, and CATV signals are transmitted and received via the input terminal 101. The CATV signals are composed of upstream signals transmitted from the cable modem to the cable line and downstream signals transmitted from the cable line to the cable modem. The upstream signals consist of signals in the range from 5 MHz to 42 MHz frequencies, and the downstream signals consist of signals in the range from 54 MHz to 860 MHz frequencies.
The upstream signals are transmitted as data signals subjected to QPSK (Quadrature phase shift keying) from a QPSK transmitter in the digital signal processing circuit and supplied to a data terminal 141. The data signals supplied to the data terminal 141 are transmitted via an LPF (Low Pass Filter) 140 as an upstream circuit from the input terminal 101 to the cable line.
Meanwhile, the downstream signals are supplied from the input terminal 101 and pass through a HPF (High Pass Filter) 102 as an IF (Intermediate Frequency) filter. Thereafter, the downstream signals are supplied to input switching circuits 103, 104, and 105. They are subjected to the respective processing in the UHF band receiver circuit, the VHF High band receiver circuit, and the VHF Low band receiver circuit. The above HPF 102 is a high pass filter having 5 MHz to 46 MHz as an attenuation band and 54 MHz or higher as a passing band. Each of the receiver circuits activates in accordance with receivable channels and does not operate for channels other than the receivable channels.
Next, the following will describe operations of the receiving bands. After passing through the signal switching circuits 103, 104, and 105, the CATV signals enter high frequency amplifying input circuits 106, 107, and 108. Then, the CATV signals are amplified in the high frequency amplifiers 109, 110, and 111, and reception signals are delivered from the high frequency amplification/output tuning circuits 112, 113, and 114. Thereafter, the reception signals are subjected to frequency conversion by mixer circuits 115, 116, and 117 and local oscillator circuits 118, 119, and 120. As described above, after signal processing in the receiver circuits, the respective output signals from the receiver circuits are IF-amplified in an intermediate frequency amplification circuit 121. Thereafter, the resultant signals are outputted from an IF output terminal 122.
The cable modem is constituted by the above-arranged tuner and a signal processing section including a QAM demodulation circuit for demodulating a signal received by the tuner. As shown in FIG. 8, the tuner is basically composed of analog circuits, while the A/D converter and the subsequent circuits in the signal processing section are composed of digital circuits. Therefore, in many cases, the tuner and the signal processing section are designed by analog circuit engineers and digital circuit engineers, respectively.
Here, the upstream signals must be amplified to a predetermined voltage to output it to the cable line. Conventionally, a return path amplification circuit for amplifying the upstream signals was provided on the signal processing section. In this case, the return path amplification circuit composed of analog circuits was designed by digital circuit engineers. However, performance in the tuner is strictly stipulated by various specifications. It is difficult for digital circuit engineers to design the return path amplification circuit so as to meet the requirements. This caused an extreme increase in time and cost.
Note that, for example, Japanese Laid-Open Patent Publication No. 14058/1993 (Tokukaihei 5-14058; published on Jan. 22, 1993) discloses a technique for improving performance of a mixer circuit in a tuner.